Water-dissolvable sodium sulfate nanowires as a versatile template for the fabrication of polyelectrolyte- and metal-based nanotubes

Article abstract of DOI:10.1021/ja063221z

This study presents the synthesis of water-dissolvable sodium sulfate nanowires, where Na₂-SO₄ nanowires were produced by an easy reflux process in an organic solvent, N,N-dimethylformamide (DMF) and formed from the coexistence of AgNO₃, SnCl₂, dodecylsodium sulfate (SDS), and cetyltrimethylammonium bromide (CTAB). Na₂SO ₂ nanowires were derived from SDS, and the morphology control of the Na₂SO₄ nanowires was established by the cooperative effects of Sn and NO₃^-, while CTAB served as the template and led to homogeneous nanowires with a smooth surface. Since the as-synthesized sodium sulfate nanowires are readily dissolved in water, these nanowires can be treated as soft templates for the fabrication of nanotubes by removing the Na₂SO₄ core. This process is therefore significantly better than other reported methodologies to remove the templates under harsh condition. We have demonstrated the preparation of biocompatible polyelectrolyte (PE) nanotubes using a layer-by-layer (LbL) method on the Na₂SO ₄ nanowires and the formation of Au nanotubes by the self-assembly of Au nanoparticles. In both nanotube synthesis processes, PEI (polyethylenimine), PAA (poly(acrylic acid)), and Au nanoparticles served as the building blocks on the Na₂SO₄ templates, which were then rinsed with water to remove the core templates. This unique water-dissolvable template is anticipated to bring about versatile and flexible downstream applications.

Full text of DOI:10.1021/ja063221z

Published on Web 08/10/2006
Water-Dissolvable Sodium Sulfate Nanowires as a Versatile
Template for the Fabrication of Polyelectrolyte- and
Metal-Based Nanotubes
†
‡
§
|
⊥
Ying-Chih Pu, Jih Ru Hwu, Wu-Chou Su, Dar-Bin Shieh, Yonhua Tzeng, and
,
†
Chen-Sheng Yeh*
Contribution from the Department of Chemistry and Center for Micro/Nano Technology
Research, National Cheng Kung UniVersity, Tainan 701, Taiwan, Department of Chemistry,
National Tsing Hwa UniVersity, Hsinchu 300, Taiwan, Department of Internal Medicine,
National Cheng Kung UniVersity, Tainan 701, Taiwan, Institute of Oral Medicine and
Department of Stomatology, National Cheng Kung UniVersity, Tainan 701, Taiwan, and
Department of Electrical Engineering and Center for Micro/Nano Technology Research,
National Cheng Kung UniVersity, Tainan 701, Taiwan
Received May 9, 2006; E-mail: csyeh@mail.ncku.edu.tw
2
Abstract: This study presents the synthesis of water-dissolvable sodium sulfate nanowires, where Na -
SO
4
nanowires were produced by an easy reflux process in an organic solvent, N,N-dimethylformamide
, SnCl , dodecylsodium sulfate (SDS), and cetyltri-
(
DMF) and formed from the coexistence of AgNO
3
2
methylammonium bromide (CTAB). Na
of the Na SO
2
SO
4
nanowires were derived from SDS, and the morphology control
-
2
4
nanowires was established by the cooperative effects of Sn and NO
3
, while CTAB served
as the template and led to homogeneous nanowires with a smooth surface. Since the as-synthesized sodium
sulfate nanowires are readily dissolved in water, these nanowires can be treated as soft templates for the
fabrication of nanotubes by removing the Na SO core. This process is therefore significantly better than
2 4
other reported methodologies to remove the templates under harsh condition. We have demonstrated the
preparation of biocompatible polyelectrolyte (PE) nanotubes using a layer-by-layer (LbL) method on the
Na
nanotube synthesis processes, PEI (polyethylenimine), PAA (poly(acrylic acid)), and Au nanoparticles served
as the building blocks on the Na SO templates, which were then rinsed with water to remove the core
2 4
SO nanowires and the formation of Au nanotubes by the self-assembly of Au nanoparticles. In both
2
4
templates. This unique water-dissolvable template is anticipated to bring about versatile and flexible
downstream applications.
3
Introduction
stepwise adsorption. Templates including porous membranes,
metal nanorods and nanowires, carbon nanotubes, and organic
Tubular nanostructured materials have drawn a lot of scientific
attention because of their diverse applications in drug delivery,
catalysis, optics, electronic, and magnetic devices. A number
substrates have been successfully employed to create tubular
structures. However, most of the template-based methods require
the use of a base or acid medium or high temperature to remove
1
of approaches have been demonstrated in preparing various
nanotubes made of metals, metal oxides, semiconductors, and
4
the templates. Considering safety and environmental hazards,
2
such processes greatly increase the cost and risk of large-scale
manufacture. In this regard, we have developed a new type of
water-dissolvable inorganic Na2SO4 nanowire, which greatly
enhances the template-based synthesis of nanotubular materials.
With regard to inorganic nanowires and nanofibers such as
sulfates, carbonates, and chromates, considerable fabrication
strategies have been restricted in utilizing reverse micelles,
microemulsion, and polymer-mediated techniques.5 Herein,
conductive polymers. Template-based synthesis is the most
widely used method, and an effective way for generating hollow
structures. Following this style of synthesis, different composi-
tions can be prepared using chemical vapor deposition or simple
†
Department of Chemistry and Center for Micro/Nano Technology
Research, National Cheng Kung University.
‡
Department of Chemistry, National Tsing Hwa University.
Department of Internal Medicine, National Cheng Kung University.
Institute of Oral Medicine and Department of Stomatology, National
-7
§
|
Cheng Kung University.
Department of Electrical Engineering and Center for Micro/Nano
Technology Research, National Cheng Kung University.
(
3) (a) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.; Yang,
P. Nature 2003, 422, 599-602. (b) Liang, Z.; Susha, A. S.; Yu, A.; Caruso,
F. AdV Mater. 2003, 15, 1849-1853.
⊥
(
1) (a) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; S o¨ derlund,
H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864-11865. (b) Liu, Z.;
Zhang, D.; Han, S.; Li, C.; Lei, B.; Lu, W.; Fang, J.; Zhou, C. J. Am.
Chem. Soc. 2005, 127, 6-7.
(4) (a) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch,
K.; Schilling, J.; Choi, J.; G o¨ sele, U. Science 2002, 296, 1997. (b) Obare,
S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601-603. (c) Mayya,
K. S.; Gittins, D. I.; Dibaj, A. M.; Caruso, F. Nano Lett. 2001, 1, 727-
730. (d) Sun, Z.; Yuan, H.; Liu, Z.; Han, B.; Zhang, X. AdV. Mater. 2005,
17, 2993-2997.
(2) (a) Jeong, J. S.; Lee, J. Y.; Cho, J. H.; Suh, H. J.; Lee, C. J. Chem. Mater.
2
005, 17, 2752-2756. (b) Martin, C. R. Science 1994, 266, 1961-1966.
11606
9
J. AM. CHEM. SOC. 2006, 128, 11606-11611
10.1021/ja063221z CCC: $33.50 © 2006 American Chemical Society

2
Na SO
4
Nanowire: Template-Based Synthesis of Nanotubes
A R T I C L E S
Scheme 1. Nanotubes Synthesis Processes Using Water-Dissolvable Na2SO4 Nanowire Templatesa
a
PE nanotubes can be fabricated either by LbL deposition or interlayer crosslinking. The Au NPTs and Au Nanotubes are produced by surface adsorption
of Au nanoparticles and the subsequent calcination, respectively.
an easy reflux process in an organic solvent is presented to form
Na2SO4 nanowires.
In this work, we also demonstrate the layer-by-layer (LbL)
templating of Na2SO4 nanowires to synthesize polyelectrolyte
cetyltrimethylammonium bromide (CTAB) in 5 mL of N,N-dimethyl-
formamide (DMF) at 160 °C for 60 min until the limpid solution turned
a milky-white color. The sample was removed from the hot plate and
cooled to room temperature. We then collected the white precipitates
by centrifugation (10,000 rpm, 5 min) and further washed them with
(PE) nanotubes, as well as the utilization of metal nanoparticles
2
-propanol several times. The derived Na
2 4
SO nanowires were stored
as the building blocks for self-assembling metal nanoparticle
tubes (NPTs). The resulting NPTs could then be further calcined
to form metal nanotubes. The metal NPTs have been reported
in 2-propanal until use.
Preparation of PE12, PE10, and PE11 Nanotubes with Internally
Embedded DNA (DNA-in-PE). The numbers in PE12, PE10, and PE11
denote the number of polymer layers. One milliliter of the prepared
8
as a new class of template synthesis of metal nanotubes. As
shown in Scheme 1, we have utilized the LbL method to assem-
ble a positively charged polyethylenimine (PEI) layer followed
by a negatively charged poly(acrylic acid) (PAA) layer in a
sequential way on the Na2SO4 nanowire surface to form Na2-
SO4 covered by PE nanowires. Alternatively, a strategy involv-
ing interlayer cross-linking via 1-ethyl-3-(3-dimethylaminopro-
pyl) carbodiimide hydrochloride (EDC) conjugation was
introduced to prepare Na2SO4 coated with PE nanowires as well.
After water rinsing, the core nanowire templates are dissolved
to obtain the PE nanotubes. Another application shown in
Scheme 1 is derived from the surface deposition of gold nano-
particles on the Na2SO4 nanowire surface to form Na2SO4@Au
nanowires and the subsequent dissolution process to yield Au
NPTs. The as-synthesized Au NPTs were able to transform into
Au nanotubes after calcination for 30 min at 300 °C.
Na
-propanol, MW: 25,000) and stirred for 2 h. PEI formed the innermost
layer of the nanotube coated on the nanowire template. The Na
SO @PEI was collected by centrifugation (12,000 rpm, 5 min), followed
by a double washing with 2-propanol to remove residual PEI. The Na
2 4
SO nanowire solution was mixed with 100 µL of PEI (1 mM in
2
2
-
4
2
-
SO @PEI was then redispersed in 1 mL of 2-propanol, added with
4
100 µL PAA (10 mM in 2-propanol, MW: 2000), and stirred for 2 h.
The Na
00 rpm, 5 min) and washing with 2-propanol. The number of layers
can be controlled by alternatively coating with PEI and PAA. The final
PE nanotubes were then obtained by simply dissolving Na SO coated
2 4
SO @PEI/PAA was thus obtained after centrifugation (12,-
0
2
4
with PE nanowires in water. The multilayer wall can be strengthened
by interlayer chemical cross-linking using 2 µL of 1-ethyl-3-(3-
dimethylaminopropyl) carbodiimide hydrochloride (EDC, 1 M in H
per mL of the nanowires. To prepare the PE11 nanotubes with internally
embedded DNA, the prepared Na SO @PEI nanowires were mixed with
0 µL of DNA-FITC (FITC-5′-ATGATCTTGATCTTCATGGA-3′
00 µM in H O) and stirred for 2 h. The resulting Na SO @PEI/DNA
2
O)
2
4
1
1
Experimental Section
2
2
4
Preparation of Na
prepared by the reflux of 0.02 M dodecylsodium sulfate (SDS,
CH (CH Na), 1 mM AgNO , 4 mM SnCl ‚2H O, and 2 mM
2 4 2 4
SO Nanowires. Na SO nanowires were
nanowires were redispersed in 2-propanol after being washed twice.
The final product of Na SO @PE11 with internally embedded DNA
2
4
3
2
)
11OSO
3
3
2
2
nanowires could be obtained following the layer-by-layer coating
process as mentioned above, and a subsequent dissolving in water
resulted in PE nanotubes containing DNA. A confocal fluorescence
microscopy image of PE11 with internally embedded DNA nanotubes
was recorded on a Leica TCS MP II system equipped with a 100× oil
immersion objective and operating in fluorescence mode. The DNA-
based PE nanotube solution was placed on a glass cover slip, and the
image of the DNA-based PE nanotubes was captured immediately.
(
5) (a) Li, M.; Mann, S. Langmuir 2000, 16, 7088-7094. (b) Qi, L.; C o¨ lfen,
H.; Antonietti, M.; Li, M.; Hopwood, J. D.; Ashley, A. J.; Mann, S. Chem.
Eur. J. 2001, 7, 3526-3532.
(6) Yu, S. H.; Antonietti, M.; C o¨ lfen, H.; Hartmann, J. Nano. Lett. 2003, 3,
3
79-382.
(
(
7) Oaki, Y.; Imai, H. Langmuir 2005, 21, 863-869.
8) (a) Lahav, M.; Sehayek, T.; Vaskevich, A.; Rubinstein, I. Angew. Chem.,
Int. Ed. 2003, 42, 5576-5579. (b) Sehayek, T.; Lahav, M.; Popovitz-Biro,
R.; Vaskevich, A.; Rubinstein, I. Chem. Mater. 2005, 17, 3743-3748.
J. AM. CHEM. SOC.
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VOL. 128, NO. 35, 2006 11607

A R T I C L E S
Pu et al.
Preparation of Au NTPs. The synthesis of Au nanoparticles was
carried out in toluene, following a previous report by Schiffrin and
9
co-workers. Briefly, an aqueous solution of HAuCl
4
(0.75 mL of 30
mM) was mixed with tetraoctylammonium bromide in toluene (2 mL
of 50 mM). The mixed solution was vigorously stirred until all Au3
ions were transferred into the toluene, followed by the slow addition
+
4
of NaBH (0.625 mL of 0.4 M). After further stirring for 20 min, the
organic phase was separated from the flask, and 100 µL of an Au
nanoparticle solution was directly added into 1 mL of the as-synthesized
2 4
Na SO nanowires solution. The mixture was incubated for 10 minutes
after sonication for seconds. The Au NPTs were then collected by
centrifugation (13,000 rpm, 10 min) and then washed several times in
2
H O. Subsequently, Au nanotubes could be fabricated after calcination
of the Au NPTs at 300 °C for 30 min.
Field emission scanning electron microscopy (Hitachi S4200) and
transmission electron microscopy (JEOL JSM-1200EX and PHILIPS
CM-200 TWIN) were performed for morphological characterization.
The X-ray diffraction (XRD) was analyzed in a Shimadz XD-D1
diffractometer using Cu KR radiation (λ ) 1.54056 Å) at 30 kV and
3
0 mA.
Results and Discussion
The sodium sulfate nanowires can be obtained by a simple
reflux of the solution containing SDS, AgNO3, SnCl2‚2H2O,
CTAB, and DMF. Upon heating to 160 °C for 1 h under
vigorous stirring, the color of the solution changed from
colorless to milky white, leading to the formation of white
precipitates. The white precipitates were washed with 2-propanol
several times and were collected using centrifugation. High-
resolution scanning electron microscopy (HR-SEM) observation
of the white precipitates is illustrated in Figure 1a. These white
precipitates were found to be one-dimensional nanowires with
diameters of 71 ( 16 nm and lengths up to 4-7 µm. The TEM
image (inset of Figure 1a) shows a solid core for these
nanowires. The X-ray diffraction (XRD) pattern indicates the
formation of crystalline Na2SO4 as shown in Figure 1b. The
as-synthesized nanowires are highly water soluble.
Figure 1. (a) SEM image of the Na2SO4 nanowires. (Inset) TEM image
of the corresponding nanowires (scale bar ) 500 nm). (b) XRD pattern of
the Na2SO4 nanowires.
The formation of sodium sulfate nanowires is intriguing. We
have schematically studied the nanowires formation. First, the
synthesis process was modified by the addition of SDS (0.02
M) to DMF only, without the presence of AgNO3, SnCl2‚2H2O,
and CTAB for reaction. The milky-white solution and the final
product of white precipitates could also be obtained. (Figure
2
,a and b) However, irregular rodlike and platelike structures
Figure 2. (a) Milky-white color was observed after adding only SDS (0.02
M) to 5 mL of DMF, after a reaction time of 1 h. (b) White precipitates
corresponding to (a) were obtained after stirring was stopped. (c) No
precipitation was observed by the addition of SDS (0.02 M) in the dried
DMF for a reaction in an argon atmosphere.
as evidenced by the SEM image were observed. The energy
dispersive X-ray spectrometer (EDS) indicates the presence of
Na, S, and O elements in the products derived (see Supporting
Information). This suggests that Na2SO4 nanomaterial can be
readily formed in a mixture of SDS and DMF solution in the
absence of AgNO3, SnCl2‚2H2O, and CTAB, although nonuni-
form rodlike and platelike structures are synthesized. Addition-
ally, if the dried DMF was used to react with SDS and the reflux
was performed in argon, no precipitate was observed (Figure
Gas chromatography measurements have confirmed a CH3(CH2)11-
OH component in the resulting solution.
FTIR characterization of as-synthesized Na2SO4 nanowires
(Figure 1a) shows evidence of DMF adsorption by the observa-
tion of the CdO signal (Supporting Information). In addition,
2
c). Apparently, the presence of H2O is important for the Na2-
the asymmetric and symmetric deformation modes pertaining
SO4 formation, and the reaction takes place as follows:
+
to CH3-N of the CTAB headgroup appear at ∼1466 and
-
1
∼
1357 cm (Supporting Information), respectively, which are
-
3
+
2
CH (CH ) OSO + 2 Na + 2 H O f
3
2 11
2
consistent with the signals of two bands at ∼1490 and ∼1390
-1 10,11
Na SO + 2 CH (CH ) OH + H SO
4
cm . This indicates the positively charged CTAB adsorbed
2
4
3
2 11
2
(
10) Li, H.; Tripp, C. P. Langmuir 2002, 18, 9441-9446.
(
9) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10,
(11) Huang, C.-C.; Yeh, C.-S.; Ho, C.-J. J. Phys. Chem. B 2004, 108, 4940-
9
22-926.
4945.
11608 J. AM. CHEM. SOC.
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VOL. 128, NO. 35, 2006

2
Na SO
4
Nanowire: Template-Based Synthesis of Nanotubes
A R T I C L E S
+
electrostatically via the headgroup N(CH3)3(C16H33) on the Na2-
SO4 surface as well. Interestingly, the ê-potential measurements
of the Na2SO4 nanowires (Figure 1a) give a -45 mV negative
surface charge of the nanowires. This suggests that the surface
of the nanowires is not fully covered, but is partially covered
by both DMF and CTAB molecules. It is worth mentioning
that the addition of less CTAB (2 mM) as compared with SDS
(0.02 M) introduced in the reaction not only allows the
functionality of CTAB to properly serve as a template and results
in homogeneous nanowires with a smooth surface but also
avoids the complete passivation of CTAB on the Na2SO4
negative surface, which provides the adsorption sites for the
further LbL approach to fabricate polyelectrolyte and self-
assembly of Au nanoparticles to yield Au nanotubes.
We have shown that Na2SO4 materials can be simply
synthesized from the solution mixture containing only SDS and
DMF. Notably, it has been known that the addition of foreign
species in the solution-phase approach facilitates the growth of
+
anisotropic structures. For example, the introduction of Ag by
adding AgNO3 has been found to assist in the growth of gold
nanorods and Pt nanocrystals with shapes varying from cubes
Figure 3. SEM images of the resulting precipitates from different
experimental conditions: (a) in the mixture containing SnCl2‚2H2O, SDS,
and CTAB; (b) in the mixture containing AgNO3, SDS, and CTAB; (c) in
the mixture containing NaNO3, SnCl2‚2H2O, SDS, and CTAB; (d) in the
mixture containing AgNO3, SnCl2‚2H2O, and SDS.
1
2,13
+
to octahedrals.
It was found that the Ag ions have acted
as a shape-control agent during the reaction. In this synthesis,
SnCl2‚2H2O and AgNO3 were added to form the high yield of
nanowires as seen in Figure 1a. The mechanisms by which both
SnCl2 and AgNO3 assist in the growth of Na2SO4 nanowires
are not understood yet. However, addition of SnCl2 was found
to particularly enhance nanowires formation. A control experi-
ment was performed by addition of SnCl2‚2H2O in the mixture
of SDS, CTAB, and DMF (without AgNO3). Following the
same reaction time (1 h), significant wire-like shapes were
generated, accompanied by particle-like products (Figure 3a).
However, only irregular particles were observed when AgNO3
was reacted with a mixture containing SDS, CTAB, and DMF
+
supernatant containing no Ag in it. Following this operation,
the reaction of the supernatant resulted in the same product
(Figure 1a) with a high yield and uniform nanowires. The other
parallel experiment was also performed by replacing AgNO3
with NaNO3. In this trial, significant nanowires with reasonable
uniformity were formed, as well as spherical-like particles
(Figure 3c). Perhaps the excessive sodium from NaNO3 might
have resulted in the spherical byproduct. Although exclusive
nanowires are not achieved using NaNO3, it is shown that the
preparation of Na2SO4 nanowires can be applicable to the
(Figure 3b). For both products obtained in Figures 1a and 3a,
EDX measurements identified the presence of the Sn signal. It
is possible that tin deposits, e.g. Sn2+ adsorption, at the specific
surface on Na2SO4, followed by a reduction of Sn2 to Sn, and
inhibits its growth, leading to preferential growth along one
direction. There were no detectable Cl (from SnCl2) and Br
-
appropriate metal nitrates, e.g. AgNO3, NaNO3, and NO3 have
a crucial role in forming such nanowire structures. Clearly, the
morphology control of the Na2SO4 nanowires is established by
+
-
the cooperative effects of Sn and NO3 . Finally, Figure 3d shows
the Na2SO4 nanowires with a rough surface, where the wire-
like structures were obtained from the mixture containing
AgNO3, SnCl2‚2H2O, and SDS in the absence of CTAB, thus
indicating that CTAB serves as a template to form nanowires
exhibiting a smoother surface. We have carried out the thermal
stability measurements of the resulting Na2SO4 nanowires by a
calcination process. Nanowires retained their shape without
apparent collapse after calcination at 500 °C for 60 min.
However, Na2SO4 nanowires began to melt and fused together
if the temperature was further increased up to 600 °C (Sup-
porting Information).
(from CTAB) signals in the EDX detection limit. Although the
introduction of SnCl2 significantly enhances one-dimensional
growth as seen in Figure 3a, additional foreign AgNO3 additive
is needed to achieve the exclusive nanowires with high
uniformity as shown in Figure 1a. Interestingly, we have
-
observed that the presence of the NO3 anion appears to be
critical, instead of silver, for generating high uniform nanowires.
Separate experiments were conducted to understand the fate of
-
the NO3 anions for nanowire formation. Since AgCl was
immediately precipitated when AgNO3 was added to the mixture
containing SnCl2‚2H2O, SDS, CTAB, and DMF, AgCl precipi-
tate was removed first by centrifugation, leaving the supernatant
for reaction. In the synthesis of Na2SO4 nanowires, it should
be noted that 1 mM of AgNO3 and 4 mM of SnCl2. ‚2H2O
An important objective achieved in this study was to utilize
the water dissolvable Na2SO4 nanowires as a versatile template
to prepare polyelectrolyte (PE) and metal-based nanotubes by
different tactics. Polyelectrolyte materials have a variety of
biomedical applications. For example, polyelectrolyte fibrous
structures can be used to serve as scaffolds for tissue engineer-
+
were used for reaction. The complete consumption of Ag is
-
expected to occur by the reaction of Cl , leading to the
14
ing. The Na2SO4 nanowires feature a surface of negative phase
-45 mV), and thus are ideal to build up tubular structures
(
12) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389.
b) Orendorff, C. J.; Murphy, C. J. J. Phys. Chem. B 2006, 110, 3990-
994.
(
(
3
(13) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. J. Phys. Chem.
B 2005, 109, 188-193.
(14) Wan, A. C. A.; Tai, B. C. U.; Leck, K.-J.; Ying, J. Y. AdV. Mater. 2006,
18, 641.
J. AM. CHEM. SOC.
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VOL. 128, NO. 35, 2006 11609

A R T I C L E S
Pu et al.
Figure 4. TEM images of (a) PE12 nanotubes. (Inset) Magnified view, scale bar ) 20 nm). (b) PE10 nanotubes with interlayer amide bond chemical
cross-link. (c) The confocal fluorescence microscopy image of the PE11 nanotube with internally embedded DNA-FITC (FITC-5′-ATGATCTTGATCT-
TCATGGA-3′) tagged with fluorescent dye.
derived by the LbL method. The LbL templating of Na2SO4
nanowires to prepare PE nanotubes was carried out utilizing
polyethylenimine (PEI) and poly(acrylic acid) (PAA) as building
blocks by a sequential deposition on the Na2SO4 nanowire
surface. The PEI provides a positively charged layer while the
PAA forms the wrapping around negatively charged layer over
the first coating of the PEI layer by layer. PEI/PAA mutilayers
were thus sequentially coated on the Na2SO4 nanowires. After
repetitive water rinsing, PE12 (12 is the number of layers) was
derived. As shown in the TEM image of Figure 4a, the PE12
formed tubelike nanostructures. These polyelectrolyte nanotubes
exhibited high flexibility and readily entangled together. Ad-
ditional TEM images taken from different views are also
provided for the purpose of giving a more comprehensive
understanding for the morphology of these polyelectrolyte
nanotubes (Supporting Information). The inset of Figure 4a
shows the open-ended structure of these PE12 nanotubes. Since
the formation of PE nanotubes relies on the interaction between
oppositely charged polymers, nanotubes can only be obtained
reproducibly when the number of the coated layers is greater
than 12 via the LbL strategy. FTIR analysis of as-synthesized
PE12 nanotubes was conducted and provided the evidence of
formation of polyelectrolyte nanotubes. Since the outmost layer
of the PE12 nanotubes was coated with PAA, the FTIR spectrum
of pure PAA polymer was determined as well (Supporting
Information). Alternatively, we are able to cross-link the polymer
layers with each other by covalent bonds when the number of
the coated layers is below 12. After coating PE10 layers on Na2-
SO4 nanowires surface, we simply added 1-ethyl-3-(3-dimethyl-
aminopropyl) carbodiimide hydrochloride (EDC) to form amide
bonds between the -NH2 of PEI and -COOH of PAA under
an incubation time of 2 h. Upon removal of the Na2SO4
nanowires by water, the PE10 nanotubes (Figure 4b) exhibited
a more rigid structure due to the covalent bond formation. We
further demonstrated that the LbL-derived nanotubes could carry
DNA by adsorption of DNA-FITC (FITC-5′-ATGATCT-
TGATCTTCATGGA-3′) over the PEI in PE nanotubes. We
replaced the second layer PAA of the PE nanotubes by DNA-
FITC in the synthesis process. Figure 4c shows the confocal
fluorescence microscopy image of the derived DNA-harbored
PE11 nanotubes.
successfully fabricated Au NPTs and Au nanotubes using the
templates of Na2SO4 nanowires. The deposition of Au nano-
particles on the surface of Na2SO4 nanowires followed by
dissolving the nanowires in water resulted in the formation of
Au NPTs in less than 10 min at room temperature. Gold
nanoparticles were synthesized in toluene using a two-phase
9
system that has been reported by Schiffrin and co-workers. The
derived Au nanoparticles in toluene have the particle size of
4.3 ( 1.2 nm and exhibit a positively charged surface in the
presence of quaternary ammonium. These Au nanoparticles were
added into the as-synthesized Na2SO4 nanowires in 2-propanol
solution, followed by an incubation of 10 minutes until the ruby-
red solution of Au nanoparticles turned into the final deep-purple
color. Subsequently, precipitates were observed and collected
by centrifugation. The Na2SO4 nanowire templates were then
removed by washing in water several times. The TEM images
of the resulting Au NPTs are shown in panels a and b of Figure
5. The positive surface charge of the Au nanoparticles adsorbed
on the Na2SO4 nanowires accompanied with coalescence to form
tubular structures, while the excess Au nanoparticles were self-
aggregated as indicated by the arrows. The inset of Figure 5b
presents the ED (electron diffraction) pattern of a single Au
NPT showing four rings, (111), (200), (220), and (311) from
center to outside, which were identified as Au. The Au NPTs
can be further calcined to 300 °C (5 °C/min) for 30 min. This
temperature is sufficient for the melting of the nanoparticles
on the surface of the Au NPTs. Figure 5c displays the resulting
Au nanotubes, where the aggregates of Au nanoparticles also
melted and formed particles with relatively larger diameters,
as indicated by the arrows. To separate Au nanoparticles from
Au NPTs or Au nanotubes, a simple centrifugation process (500
rpm, 1 min) was operated twice and removed 50% of particles.
A higher centrifugation rate did not apparently improve ef-
ficiency to exclude particles since the resulting nanotubes in
length can be readily longer than 1 µm. Size exclusion was also
performed by filtration using filter papers with pore sizes ∼1
µm and could achieve an efficacy of about 60-70% in
separation (Supporting Information). A more efficient separation
method needs to be developed and is currently underway.
Conclusions
Metal nanoparticle tubes (NPTs) have been categorized as a
new class of template-synthesized nanomaterials. We have also
We have successfully synthesized sodium sulfate nanowires
by a simple solution approach. The as-synthesized Na2SO4
8a
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2
Na SO
4
Nanowire: Template-Based Synthesis of Nanotubes
A R T I C L E S
Figure 5. (a) TEM image of the Au NPTs and Au nanoparticle aggregates (arrows). (b) TEM image with high magnification of a single Au NPT. (Inset)
ED pattern of the corresponding nanotubes). (c) Au nanotubes derived from the calcination of the Au NPTs at 300 °C for 30 min accompanied by large Au
nanoparticles as indicated by the arrows. (Inset) High magnification of a single nanotube (scale bar ) 20 nm).
Acknowledgment. We thank the National Science Council
of Taiwan for financially supporting this work.
nanowires can be readily dissolved in water. We also demon-
strated that the Na2SO4 nanowires served as a water-removable
template for the fabrication of PE and Au nanotubes using the
LbL method and particle self-assembly, respectively. This
versatile new nanotube-synthesis protocol based on water-
dissolvable Na2SO4 nanowires holds great potential for innova-
tive composite materials and biomedical applications such as
artificial genes or tissue engineering.
Supporting Information Available: TEM and SEM images
and the analysis of EDX and FTIR spectra for the relevant Na2-
SO4 nanomaterials, TEM images and FTIR characterization for
PE12 nanotubes, and TEM view for Au NPTs. This material is
available free of charge via the Internet at http://pubs.acs.org.
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